Hey Giorgio,
the reason Bitcoin has gained so much popularity in the dark web is because of its pseudonymous nature which makes transactions hard to track back to their owner. That’s why a lot of criminals prefer this form of currency. Of course anonymity is not the only trait of Bitcoin, that’s why a lot of legit users have adopted it as well.

I’ve heard about Monero although I didn’t do a complete drill down yet. I think Bitcoin has a 7 year track record that Monero will need to beat and that’ not easy. There have been so many hypes about different altcoins in the past years that I don’t pay attention that much whenever a new coins arrises until it gains significant market cap.
So while Monero can be a competitor at the moment it’s not in the same league.